Keyboards or keypads are used to provide user input to a variety of devices, including portable electronic devices. Handheld electronic devices, such as cellular telephones, personal digital assistants and handheld computers for example, are battery operated. It is highly desirable that these devices have low power consumption so as to maximize battery life.
Keyboard readout is the process of detecting a key switch and generating a signal to a processor to indicate which key has been pressed.
One approach to keyboard readout is to use one wire for each key of the keyboard. Each wire is coupled to an input of a processor and the processor monitors the inputs to detect when a key is pressed. The approach requires a relatively large number of processor input pins (for example an 8×8 keyboard would require 64 pins) which tends to increase the size and cost of the device. Additionally, the processor has the burden of monitoring these input pins. The action of monitoring the inputs produces a load on the battery and shortens battery life.
A further approach to keyboard readout is to monitor the keys in an array or matrix. In this approach, one output line is used for each row of the matrix and one input line is used for each column of the matrix (for example an 8×8 keyboard would require 8 output lines and 8 input lines). A logic signal is sent to each output line in turns and all inputs lines are monitored. FIG. 1 is a diagrammatic representation of keyboard readout apparatus that uses this approach. FIG. 1 shows a keyboard or keypad 100 consisting of 16 key switches 102 arranged in a 4×4 rectangular array or matrix. Scan lines 104, 106, 108 and 110 allow the scan signals shown in plot 112 to be coupled to the rows of the rectangular array. Each columns of the rectangular array has a sense line (114, 116, 118 and 120) that is coupled via a resistor 122 to an electrical source 124.
In operation, when a key switch is activated, the corresponding scan line is electrically coupled to the sense line. For example, when the black key in FIG. 1 is pressed, the SCAN 2 line 108 is coupled to the SENSE 2 line 118. This causes the signal on the SENSE 2 line 118 to follow the signal on the SCAN 2 line 108, as shown in the plot 126. None of the other sense lines is coupled to a scan line, so their signals remain high and do not follow any scan line. The activated key is identified uniquely by monitoring the scan lines and comparing the timing of any signals to the scan signals. This process requires processing power for both the generation of the scan signals and the monitoring of the sense signals.